I have the 47BG of B18s in San Francisco - 21 Dec 41 - and was checking through the air groups to withdraw and noticed that I do not have the option for these planes ? Needless to say plenty of supply at base and not that far from air HQ (Fourth USAAF)

They are listed as as [S][R] - the [R] being restricted - I don't recall what the [S] represents - static maybe ?

Assuming you mean that when you click the upgrade button you are not given any other aircraft model to swap with the current B18, the answer is that is WAD. Not every air unit is allowed a different model to change to. That is set up by the scenario designer. You would have to go into the editor to change the upgrade path.

If you are provided with an alternative aircraft model but the game does not allow you to effect the change, that is because the unit is scheduled to withdraw. Such units must have sufficient planes in the pool to full re-equip the unit. You cannot partially re-equip a unit scheduled to withdraw, but you can do so to a unit which is not scheduled to withdraw.

Sorry I was not more clear - these are groups that must be withdrawn by 15 Jan 42. The Disband button is available but the Withdraw is greyed (grayed ?) out. It makes sense that these groups can only be disbanded. (They have 1 plane/1 pilot)